In a wireless sensor network, communication between nodes is generally performed using a broadcasting method.
In the related art, reference broadcast synchronization (RBS) has been suggested as a time synchronization method in which energy efficiency is improved on the basis of characteristics of a wireless sensor network and error elements occurring at the time of applying a time difference calculation method through a timestamp message exchange are considered. In the RBS, nodes receive reference signals that are broadcast on a wireless sensor network, and use arrival points of time of the reference signals at the time of time comparison with other nodes to perform time synchronization between receiving nodes. In the RBS, when the reference signals are broadcast, a time difference should be calculated and maintained with respect to all of receiving nodes that receive the reference signals, which results in increasing the amount of messages used when time information is exchanged between the nodes. That is, when broadcasting is generated m times with respect to N nodes, the number of messages used during a synchronization process becomes O (mN2). As a result, energy efficiency is lowered. In order to provide a time synchronization method in a multi-hop environment, a node that commonly receives a message between two reference nodes should convert time of one area to another area. As a result, when the number of hops increases, time synchronization precision decreases.
As another time synchronization method, a timing-sync protocol for sensor networks (TPSN) is used. According to the TPSN, the operation is made in two steps, that is, a level setting step and a time synchronization step, in such a manner that a transmitter corrects its time on the basis of time of a receiver. Specifically, in the first step, a level is allocated to each of nodes on the network to form a hierarchical topology, and in the second step, a low-level node performs time synchronization on a high-level node. Finally, all nodes are synchronized with an uppermost node, and time synchronization is performed over the network. The TPSN provides much better performance than that of the RBS in a multi-hop environment, but is disadvantageous in that it is not possible to use a linear regression method that is used in the RBS in order to correct clock skew. Further, the TPSN cannot efficiently adjust to a dynamic change in a topology, and is not good in terms of scalability and a fault-tolerant system.
As another time synchronization method, a flooding time synchronization protocol (FTSP) may be used. The FTSP is used to synchronize local clocks of all nodes on a network. In the FTSP, time information is transmitted through flooding and a receiver receives the transmitted time information. The receiver corrects time errors on the basis of previously analyzed error elements and a linear regression analysis. The FTSP has a merit in that it can dynamically cope with a topology change due to a defective node and communication interruption, through flooding of a periodical synchronization message. However, the error elements that have been analyzed in the FTSP become different whenever an applied system is changed. These features affect accuracy of synchronization, which makes it difficult to generally apply the FTSP.
Accordingly, it is required to develop a new time synchronization method that can reduce a synchronization error and cope with various topology changes. Further, a time synchronization method that can overcome collision due to network congestion and reduce the amount of time needed until all nodes are synchronized is required.
Meanwhile, the time synchronization methods according to the related art have a drawback in security. Particularly, according to the related art, a method of efficiently coping with a node capture attack has rarely been studied. Due to the node capture attack that is a local attack, all information of the captured node that is related to encoding and decoding keys, an authentication key, routing, and contents stored in a memory is exposed to an invader. It is almost impossible to prevent the node capture attack and it is difficult to detect the node capture attack.
If the node capture attack occurs at the time of performing time synchronization using the RBS, erroneous time information is transmitted from the captured node to a certain node. As a result, the certain node that has received the erroneous time information from the captured node may erroneously calculate clock skew and offset, and makes it difficult to perform time synchronization on an entire network due to transmission of erroneous clock conversion information.
If the node capture attack occurs at the time of performing time synchronization using the TPSN, a parent node responds to a time synchronization request transmitted from a child node at an erroneous transmission/reception time. As a result, it may become impossible to accurately perform time synchronization on all lower nodes that form a spanning tree. The case may also be generated in which the captured node informs false level information and behaves as a false parent node.
In the case in which the node capture attack occurs at the time of performing time synchronization using the FTSP, if the captured node behaves as a root node, packets that start having a sequence number higher than that of the root node are transmitted, which makes it difficult to perform time synchronization over an entire network.
Accordingly, in order to prevent a Byzantine failure due to the node capture attack, it is required to develop a time synchronization method that can determine the captured node and prevent the node capture attack.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.